1. Field of the Invention
The present invention deals with composite elements having improved resistance against stress cracking corrosion, comprising:
(A) at least one top layer preferably from a toughened polystyrene; and PA0 (B) a layer of preferably polyurethane rigid foam, whereby said rigid foam is prepared while using difluorochloromethane or a mixture of water, difluorochloromethane, and optionally trichlorofluoromethane as a blowing agent. PA0 (A) at least one covering layer of polystyrene, preferably toughened polystyrene; and PA0 (B) a layer of polyurethane rigid foam, preferably polyurethane rigid foam; PA0 (A) Polystyrenes, preferably toughened polystyrenes, having the following mechanical properties are used in the preparation of said top layer (A): PA0 a tensile strength according to DIN 53455 of from 20 to 45 N/mm.sup.2, more preferably 24 to 32 N/mm.sup.2; PA0 a percentage elongation at break according to DIN 53455 of from 15 to 45 percent, more preferably 35 to 42 percent; PA0 a modulus of elasticity (tensile test) according to DIN 53457 of from 1650 to 2800 N/mm.sup.2, more preferably 1650 to 2000 N/mm.sup.2 ; PA0 a flexural strength according to DIN 53452 of from 36 to 78 N/mm.sup.2, more preferably 36 to 50 N/mm.sup.2; PA0 an impact strength according to DIN 53453 at 23.degree. C. without break from 65 kJ/m.sup.2 and larger; more preferably at -40.degree. C. without break up to 65 kJ/m.sup.2 and greater; and most preferably 58 to 65 kJ/m.sup.2 ; and PA0 a notched bar impact strength according to DIN 53453 of from 4 to 10.5 kJ/m.sup.2, more preferably 7 to 9 kJ/m.sup.2, whereby the mechanical properties are measured on injection molded, normal small bars or on shoulder bars according to DIN 53455. Polystyrenes having selected mechanical properties are among the commercial trade products. PA0 (B) As already established the polyurethane foam layer (B) is preferably a polyurethane foam intermediate layer and most preferably the hollow space of the molded article is foamed using a polyurethane rigid foam which is prepared by reacting: PA0 (bi) 0 to 95 parts by weight, more preferably 20 to 80 parts by weight of a polyether polyol initiated with sucrose having a hydroxyl number of from 300 to 500, more preferably 350 to 450, based on 1,2-propylene oxide or 1,2-propylene oxide and ethylene oxide; PA0 (bii) 0 to 15 parts by weight, more preferably 5 to 15 parts by weight of a polyether polyol initiated with sorbitol having a hydroxyl number of from 400 to 600, more preferably 450 to 550, based on 1,2-propylene oxide or 1,2-propylene oxide and ethylene oxide; PA0 (biii) 0 to 20 parts by weight, more preferably 5 to 15 parts by weight of a polyether polyol initiated with ethylenediamine having a hydroxyl number of from 700 to 850, more preferably 750 to 800, based on 1,2-propylene oxide; and PA0 (biv) 0 to 60 parts by weight, more preferably 5 to 40 parts by weight of a polyether polyol having a hydroxyl number of from 400 to 600, more preferably 450 to 550, based on 1,2-propylene oxide or 1,2-propylene oxide and ethylene oxide prepared while using a mixture of sucrose and triethanolamine in a weight ratio of from 1:2 to 2:1 as an initiator molecule. PA0 (ei) 0.05 to 4.0 parts by weight, more preferably 0.1 to 3.5 parts by weight of water; PA0 (eii) 5 to 30 parts by weight, more preferably 8 to 20 parts by weight of difluorochloromethane; and PA0 (eiii) 0 to 30 parts by weight, more preferably 0 to 20 parts by weight of trichlorofluoromethane.
2. Description of the Related Art
The preparation of composite elements made from a polyurethane rigid foam (polyurethane is henceforth abbreviated PU) and at least one top layer of a rigid or elastic material is well known. Such composite elements are, for example, insulation panels having top layers of kraft paper, asbestos paper, or crepe paper, bituminized paper, polyethylene coated glass non-wovens, and aluminum foils; construction elements having dual sided metal covering layers from, for example, painted or coated steel sheets or aluminum sheets; or combination plaques having a top layer of a rigid panel such as, for example, a backing panel, a gypsum panel, a cardboard panel, a glass fiber panel, a rock wool panel, or a perlite panel, and a top layer from, for example, bitumen paper, or a glass non-woven material. Typical blowing agents used to prepare the polyurethane rigid foams which are suitable as an intermediate layer are preferably trichlorofluoromethane and dichlorodifluoromethane optionally used in combination with carbon dioxide formed from the reaction of an isocyanate with water.
Also known is foaming hollow spaces in household appliances such as, for example, refrigerators or hot water tanks using polyurethane rigid foam as a heat insulating material. In order to avoid cavities or pockets in the isolated hollow area, the foamable polyurethane reaction mixture must be injected into the hollow space within a short time, for example, 5 to 10 seconds. To foam such household appliances, low pressure machines or preferably, high pressure machines, are used.
Typical insulating polyurethane rigid foams can be prepared conventionally by reacting organic polyisocyanates with one or more higher molecular weight compounds having at least 2 reactive hydrogen atoms, preferably polyester polyols and/or polyether polyols optionally in conjunction with lower molecular weight chain extending agents and/or crosslinking agents, in the presence of catalysts, blowing agents, and optionally auxiliaries and/or additives. By properly selecting the starting components, one can obtain polyurethane rigid foams having very low coefficients of thermal conductivity. Such foams, even at low densities, possess excellent mechanical properties.
As previously stated, trichlorofluoromethane is preferably used as the blowing agent in the preparation of the insulating polyurethane rigid foams. The inside housing and door jacketing of refrigerators and the inside cover of freezer chests generally is made of toughened polystyrene. This material has a disadvantage in that it is not resistant to liquid or gaseous trichlorofluoromethane. Even after short term contact with gaseous trichlorofluoromethane from the foaming reaction mixture which forms the polyurethane rigid foam, the deep drawn wall segments of toughened polystyrene can be damaged on account of their inner stress or tension. In order to avoid so called stress cracking corrosion, combination panels of toughened polystyrene are frequently used for the deep drawn parts, which are protected by a thin ABS film or polyethylene film on the side facing the polyurethane rigid foam. Another disadvantage is that there is little adhesion between the housing portion of toughened polystyrene and the polyurethane foam which diminishes the mechanical strength of the molded part produced.
A comprehensive overview concerning the preparation of composite elements while using polyurethane rigid foams as covering layers or preferably core layers, as well as using polyurethane rigid foams in household appliances or in low temperature technology has been published in Polyurethanes, Plastics Handbook, Volume 7, Second Edition, 1983, edited by Dr. Gunter Oertel, Carl-Hanser Publishers, Munich, Vienna, pages 250 ff.
In addition to trichlorofluoromethane previously cited other physically effective blowing agents can be used in the preparation of polyurethane rigid foams. One example is found in DE-C-1 045 644 (USA 3 391 093) which discloses gaseous hydrocarbons having not more than 3 carbon atoms, such as methane, ethane, ethylene, propane, and propylene, and halogenated hydrocarbons such as, for example, chloromethane, dichlorodifluoromethane, dichlorofluoromethane, chlorodifluoromethane, chloroethane, and dichlorotetrafluoroethane as well as octafluorocyclobutane and hexafluoropropane. Another example is found in Belgian Patent 596 608 which discloses halogen alkanes such as, for example, 1,1-difluoro-2,2-dichloroethane, 1,2-difluoro-1,2-dichloroethane, 1,1-dichloroethane, 1-fluoro-1,2-dichloroethane, 1-fluoro-2,2-dichloroethane 1,2-dichloroethane, trichloroethane, tetrachloroethane, 1-fluoro-2,2-trichloroethane, bromomethane, and 1,1,2-trifluoro-2-chlorethane.
The aforesaid blowing agents have somewhat of a disadvantage in that they are toxic and/or combustible, or compared to trichlorofluoromethane they possess a lower gas yield when blowing polyurethane foam because of their boiling point, or they make the polyurethane foam have a lower insulating effect and finally they also cause stress cracking corrosion.
To avoid stress cracking corrosion on toughened polystyrene, FR-B-1 564 594 discloses the preparation of refrigerators while using trichlorofluoroethane (F 113) and dichlorotetrafluoroethane (F 114). These perhalogenated hydrocarbons, however, like trichlorofluoromethane are suspected of damaging the ozone layer and their use is prohibited according to the Montreal Protocol regarding protection of the ozone layer. Another blowing agent is carbon dioxide, which according to GB-A-21 16 574 can be dissolved under pressure in at least one starting component to prepare polyurethane rigid foam; said carbon dioxide can be thermally cleaved from salts such as, for example, carbamates; carbonates, such as, for example, ammonium carbonate; or bicarbonates, or it can be formed from the reaction of isocyanate with water to form urea groups. Along with established industrial processing difficulties when using solid carbon dioxide or gaseous carbon dioxide under pressure, this method of preparing polyurethane rigid foams also has a significant disadvantage in that the thermal conductivity of the carbon dioxide is twice as high as that compared to trichlorofluoromethane and the result is that the foams have a lower insulation value which at present is generally unacceptable.
The object of the present invention was to develop composite elements especially suited for low temperature housing compartments, having improved resistance against stress cracking corrosion whereby the aforesaid disadvantages would be overcome particularly with respect to the toxicological and environmental hazards of the blowing agent in the insulating material and/or in the preparation of the insulating material.
This object was surprisingly met by using toughened polystyrene in combination with polyurethane foams prepared while using difluorochloromethane.
Accordingly, the subject invention pertains to composite elements having improved resistance against stress cracking corrosion, comprising:
wherein difluorochloromethane is used as the blowing agent in the preparation of the polyurethane foam.
The subject invention further pertains to using such composite elements for low temperature housing compartments according to claim 8.
The difluorochloromethane suitable as a blowing agent in the preparation of the polyurethane foam according to the present invention has an advantage in that it is non-toxic and non-combustible. Moreover, as a commercial product it is available at low prices in large quantities. The thermal conductivity coefficient of difluorochloromethane is indeed greater than that of trichlorofluoromethane, however, clearly smaller than that of carbon dioxide. Another advantage is that gaseous difluorochloromethane having a boiling point of -40.8.degree. C. is very easily soluble in the polyhydroxyl compounds used in the preparation of polyurethane foams. The solubility, for example, is clearly greater than that of gaseous dichlorodifluoromethane having a boiling point of -29.8.degree. C. which is used in special processes in small quantities as a pre-foaming agent in the preparation of polyurethane rigid foams. The solubility of difluorochloromethane depending on the type of polyurethane foam formulation at 20.degree. C. is from 4 to 6 weight percent, based on the weight of the polyhydroxyl compound, per bar of pressure. If ozone layer damage should occur by the difluorochloromethane, then this would be smaller by a factor of 20, quantity related, compared to trichlorofluoromethane whereby this advantageous effect would even be further reinforced through the higher gas yields when foaming.